In wavelength-division multiplexed (WDM) fiber-optic systems, multiple wavelengths are carried on a common fiber such that information on the wavelengths can maintain their separate integrity and be separated only before conversion from optical to electronic format. Currently, backbone point-to-point links have been implemented using multiple wavelengths to increase the fiber information-carrying capacity. For a WDM wavelength-routing network, which has a topology beyond a simple point-to-point link, it is desirable that per-wavelength routing be done at the nodes of the network (P. E. Green, "Optical Network Update," IEEE Journal on Selected Areas in Communications, vol. 14, pp. 764-779, June 1996). This has the benefits of reducing the equipment required to detect, process and re-transmit information which was intended to be passed on to the next node. To minimize handling, it is desirable to keep the information on its original wavelength or optical carrier, and to route it using optical components which distinguish based on wavelength. It is further desirable that the routing be dynamically reconfigurable such that any one wavelength could be re-routed without interruption to any other wavelength which may be already carrying information.
The simplest form of purely optical wavelength-routing node is an optical add-drop multiplexer (OADM) with single input and output fibers, in which incoming data may either be passed through the node or dropped to a local receiver. In purely optical wavelength routing, all the data on a particular wavelength per fiber is considered an inseparable data stream. If data from a particular wavelength is dropped, this wavelength is now available on the outbound direction, and hence new data can be added from a local transmitter. More complex nodes may include multiple input and output fibers that data streams need to be routed between.
There have been various proposed implementations for a wavelength-routing node, particularly an OADM. Implementations related to the present invention are described in U.S. Pat. No. 5,448,660, issued Sep. 5, 1995, entitled "Wavelength Selective Optical Switch", and U.S. Pat. No. 5,479,082, issued Dec. 26, 1995, entitled "Device for Extraction and Re-insertion of an Optical Carrier in Optical Communications Networks" both by Calvani et al. Calvani discloses a tunable optical bandpass filter and 1.times.1 add-drop multiplexing network. Here, there are at least two circulators per wavelength. Thus, amplifiers may be needed to compensate for losses as the number of wavelengths increases.
The principle of using reflection filters in combination with a circulator to separate WDM signals is described by L. Quetel, et al. in "Programmable Fiber Grating Based Wavelength Demultiplexer," presented at OFC, Feb. 28, 1996, paper WF6.
Many OADM implementations operate by demultiplexing all the wavelengths and then performing space-switching on a per-wavelength basis. Then, the wavelengths to be passed (rather than dropped) are re-multiplexed onto the output fiber. There are various well-known wavelength-selective devices which can implement a de/multiplexer. The de/multiplexer can be implemented by low-loss wavelength-selective devices such as gratings. The multiplexer also can be implemented by simple broad-band wavelength combiners such as a star coupler, at the expense of higher loss. For example, the architecture reported by P. A. Perrier, et al. ("4-Channel 10-Gbit/s Capacity Self-healing WDM Ring Network with Wavelength Add/drop Multiplexers," presented at OFC, Feb. 29, 1996, paper ThD-3) uses a star coupler and tunable transmission filters to separate the wavelengths prior to the space switch, and then another star coupler to re-multiplex. Other architectures use integrated waveguide grating devices to demultiplex and multiplex with minimal loss. See, for example, H. Toba, et al., "An Optical FDM-based Self-healing Ring Network Employing Array Waveguide Grating Filters and EDFA's with Level Equalizer," IEEE Journal of Selected Areas in Communications, vol. 14, pp. 800-813, June 1996. See also U.S. Pat. No. 5,488,500, issued Jan. 30, 1996, entitled "Tunable Add Drop Optical Filtering Method and Apparatus", by B. Glance.
There is particular interest in implementing dual-fiber ring topologies with wavelength routing network nodes to provide self-healing capability as described by both Toba and Perrier. In this case, it is essential that the signal can traverse the entire ring at least once and possibly wrap around almost two times. A SONET supports up to 16 nodes in a ring. Clearly, extensions to as many nodes as possible is highly desirable.
One key limitation in optical wavelength routing as described above is the number of nodes that can be traversed without unacceptable corruption of the information. Consider a given wavelength as it passes through successive nodes. By necessity, the demultiplexing component has a limited optical bandwidth. If used, a wavelength-selective multiplexer also has a limited bandwidth. Passing through multiple components successively narrows the effective optical passband experienced by that wavelength. Eventually, the passband may become so narrow that distortion to the information results. In the best case, the distortion occurs when the passband is comparable to the information bandwidth, but in practice it may happen much sooner. Inaccuracies in the center frequency or width of each component passband, carrier wavelength fluctuation and tolerance are factors. See, for example, N. N. Khrais, et al., "Effect of Cascaded Misaligned Optical (De)multiplexers on Multiwavelength Optical Network Performance," presented at OFC, Feb. 29, 1996, paper ThD-4. It is thus highly desirable to minimize spectral filtering effects in a wavelength-routing OADM network node. The present invention addresses such a need.
Another potential problem of the OADM implemented by conventional methods is the unpredictable results on wavelength routing upon node power failure. In some cases, the input signals can be totally lost due to, for example, the walk-away of the tunable filter in the configuration described by Perrier. The present invention addresses such a need.